As is well known in the art, semiconductor devices are widely used in various electronic components and devices such as transistors, integrated circuits, lasers, and the like. It is also well known that the passage of current through a junction results in a certain amount of power loss and heat generation therein. Continuous operation or frequent activation with minimal off periods may result in elevating the junction temperature. This elevated temperature can cause two problems. First, some integrated circuit (IC) devices are susceptible to drift (e.g., lasers). A temperature that is a function of loading can be a source of drift that is difficult to predict. Second, many devices such as microprocessors have a high number of junctions per volume. This results in devices that have very high power densities and are susceptible to overheating. This overheating may result in the failure of the semiconductor to perform its assigned circuit function and may sometimes involve the destruction of the semiconductor device itself. It is therefore important to monitor the junction temperature and perform a control or alert function based upon the results.
Prior art has taught several different methods of determining the junction temperature of IC devices.
1. Temperature sensing directly on the die of the IC. PA1 2. Recreation of the IC into a more thermally predictable device. PA1 3. Pure computational methods (no sensing). PA1 4. Single point temperature sensing remote to the die of the IC (e.g., on the package of the IC) followed by an extrapolation of the junction temperature. PA1 5. Single point temperature sensing remote to the die of the IC (e.g., on the package of the IC) and a measurement of the ambient temperature prior to extrapolation of the junction temperature. PA1 to provide for an accurate and cost effective temperature sensing method for the junction temperature of semiconductor devices; PA1 to provide a method of junction temperature sensing that is insensitive to the configuration or quality of the thermal solution; PA1 to provide a method of junction temperature sensing that is insensitive to changes in the configuration or quality of the thermal solution throughout the life of the device; PA1 to provide a method of junction temperature sensing that is compatible with existing components. These existing components may or may not have an integrated temperature sensor; PA1 to provide a method of junction temperature sensing only where it is thermally necessary; PA1 to provide a method of junction temperature sensing that is insensitive to changes in the environment surrounding the component or its thermal solution(s); PA1 to provide a method of junction temperature sensing that can be an input to a control circuit used to regulate the same junction temperature; PA1 to eliminate the burdening of the design and manufacture of a semiconductor device with an integrated temperature sensor; and PA1 to provide a signal relating to the heat flux through a particular thermal path. Changes to this value, relative to the total heat flux being dissipated, are evidence of the state of the thermal dissipation means.
Sensing on the die itself is accomplished in a variety of different ways. Some proposals (e.g., U.S. Pat. No. 5,639,163-Davidson et al., U.S. Pat. No. 5,555,152-Brauchle et al., U.S. Pat. No. 5,422,832-Moyal, U.S. Pat. No. 5,291,607-Ristic et al., U.S. Pat. No. 3,383,614-Emmons et al., U.S. Pat. No. 4,896,199-Tsuzuki et al., U.S. Pat. No. 5,406,212-Hashinaga et al., and U.S. Pat. No. 5,206,778-Flynn et al.) include the use of a monolithically integrated environmental sensor. One typical implementation of this environmental sensor is a pair of on-chip thermally responsive diodes coupled to a remote current source. The diode pair generates differential voltage output proportional to temp. Other proposals (e.g., U.S. Pat. No. 4,896,245-Qualich, U.S. Pat. No. 3,521,167-Umermori et al., U.S. Pat. No. 4,970,497-Broadwater et al., and U.S. Pat. No. 4,039,928-Noftsker et al.) rely on the fact that the impedance of internal circuitry varies as a function of temperature. Similarly, these circuits are driven by an external source and the resulting voltage drop is correlated to junction temperature. Despite the theory, in practice, the output of these circuits varies from one manufactured on-chip circuit to another to an extent that calibration particular to each on-chip circuit is required. Differences in construction and operation between sensors and semiconductor devices such as microprocessors have led the semiconductor industry to shun their integration into the same substrate. In operation, most sensors generate analog signals that have been difficult to process in digital microprocessors. Interface circuits used to couple the analog sensor signal to a microprocessor require additional semiconductor devices and further discourage monolithic integration of sensors and microprocessors. In addition, the inclusion of a sensing circuit on an IC die naturally results in a larger die to be fabricated. The manufacturing yield of devices such as microprocessors is inversely proportional to die size. Thus, the inclusion of sensing circuits into an IC result in a circuit that is more difficult to manufacture. Another difficulty with this type of technique is that the solution must be designed into a particular device. Devices already in existence cannot be sensed with this technique since they do not have the sensor on the substrate. Even if sensors are monolithically integrated onto the substrate, some environments that those components go into may be thermally challenging while others may not. Even if the environment is not thermally challenging and no sensing or control is required, the purchaser of this device to be used in this environment is still burdened with the extra cost and size of these devices.
Squires (U.S. Pat. No. 3,502,944), Demarest et al. (U.S. Pat. No. 4,117,527), and Barker et al. (U.S. Pat. No. 4,669,025) teach methods for recreating or simulating the thermal condition of the IC in question into a different form. The goal with these techniques is to overcome the shortcomings of on-die measuring mentioned above. This simulated IC is monitored and the control circuit drives the actual IC in response to the thermal state of the simulated IC. In practice, this simulation is very difficult to achieve. The actual IC and the simulated IC cannot occupy the same space. Therefore, the simulated IC and the actual IC are operating in different environments. Often in electronic devices, the environment can vary greatly between chips that are even right next to each other. In addition, if a thermal solution (heat sink, heat pipe, heat spreader, peltier junction, etc.) is imposed upon the IC of interest, the same thermal solution must be imposed on the simulated IC. Besides generating additional cost and complexity, an identical thermal solution is difficult to achieve primarily because of thermal impedances across interfaces of different materials. In other words, an IC and a corresponding simulated IC can be attached to identical heat sinks. The surface roughness of the components and heat sinks at the interfaces can vary. In addition, the forces clamping the heat sink to the IC may not be identical to the forces clamping the heat sink to the simulated IC. These and other factors can contribute to thermal impedances that are not identical between the component in question and its simulation. These differences can be dramatically reflected in the output making the simulation inaccurate. Even if the simulation and the actual component are reasonably similar, the solution is still problematic as the simulation generates additional heat. Thus, the overall heat generated by the system is greater than the heat generated by the component itself. The total heat is a combination of the component and its simulated counterpart. This can have the adverse effect of overheating the component or other nearby components or devices.
Kenny et al. (U.S. Pat. No. 5,287,292) and Chen et al. (U.S. Pat. No. 5,422,806) teach a method of sensing and control that does not involve temperature sensing at all. These methods monitor how a device is driven during usage. In theory, by integrating this usage over time, a prediction can be made as to the current state of the junction temperature. In practice, it is very difficult to produce accurate results with an accumulated operating history because the local environmental conditions are not known. Even if the ambient conditions are well known at one point in time, they can change rapidly and dramatically. Condition changes of this type may include the sudden loss of system cooling capability, the unanticipated obstruction of the flow of device cooling medium, or the repositioning of the device. other changes over the life of the device may relate to the quality of the thermal impedances across interfaces. The thermal impedance of many thermal interface materials such as thermal greases decreases over the first few days of the life of a system. The impedances of these or other interfaces may increase over the life of a device because of handling. All of these changes can result in a device with an unknown or highly variable heat transfer capability.
The most common method of sensing and control currently in practice is the presence of a temperature sensor external to the die of the IC. Examples of this configuration can be seen in U.S. Pat. No. 5,664,201-lkedea, U.S. Pat. No. 3,906,310-Esashika, U.S. Pat. No. 3,688.295-Tsoras et al., U.S. Pat. No. 4,001,649-Young, U.S. Pat. No. 4,330,809-Stanley, U.S. Pat. No. 4,689,659-Watanabe, U.S. Pat. No. 5,008,736-Davies et al., U.S. Pat. No. 5,119,265-Qualich et al., U.S. Pat. No. 5,230,055-Katz et al., U.S. Pat. No. 5,230,074-Canova et al., U.S. Pat. No. 5,345,510-Honda, U.S. Pat. No. 5,477,417-Ohmori et al., U.S. Pat. No. 5,600,575-Anticole, U.S. Pat. No. 5,618,459-Kamiya, U.S. Pat. No. 5,712,802-Kumar et al. and U.S. Pat. No. 5,763,929-lwata. In these designs, a temperature is measured at a point X. A term commonly called .theta..sub.JX is utilized to convert this temperature to a junction temperature. The dimensions of .theta..sub.JX are Temperature/Power and the units commonly referenced in manufacturer's data sheets are .degree. C./Watt. Manufacturers of IC's often publish values for .theta..sub.JX where the X is replaced by C for case temperature and A for ambient temperature. The problem with this solution is that it assumes that all the heat flux from the component flows through the location X and the temperature of X is one definable temperature. For example, consider a component where the temperature measurement occurs on the case of the device. .theta..sub.JX is listed at 5.degree. C./W and the power delivered to the device is measured at 10W. If the maximum junction temperature, .theta..sub.JX, for this particular device is rated at 100.degree. C., then the device will go over temperature when the case measures 50.degree. C. This may be close to accurate if the vast majority of the heat flux travels through that particular portion of the case. This may be a reasonable assumption if, for example, a heat sink was also attached to the case and a fan passed cooling medium across the heat sink. However, consider if instead of being attached to the case, the heat sink device was attached to the backside of the printed circuit board (PCB) opposite the device. A large portion of the heat will be conducted through the PCB and passed to ambient via the heat sink instead of through the top case of the IC. In this situation, there will be a very small thermal gradient between the junction and the top case since the majority of the 10W are traveling the other direction. When the case reads 50.degree. C., T.sub.j is actually not much higher that 50.degree. C. Thus, T.sub.j would be dramatically overestimated in the later case compared to the former. Another problem with this solution is that the above inventions assume that .theta..sub.JX is known and does not change over time. In fact, .theta..sub.JX can change over time. The thermal impedances across interfaces may change over time due to handling or other environmental factors. Thermal greases and some interface materials used between devices and heat sinks actually decrease their thermal impedances over the first few days of their usage. In situations where X is the ambient temperature, .theta..sub.JA it is imperative that anywhere heat flux flows from the IC's thermal solution to ambient, the ambient temperature must be uniform. In practice, this is improbable. Orientation of the unit, neighboring heat producing components, cooling medium currents, external radiant energy (sunlight), etc., all contribute to making the ambient temperature a very complex function of space. Therefore, except in the most ideal conditions, T.sub.j will not be accurately estimated using this technique. In some cases, the cooling medium may be very uniform. Even in these cases, ambient measurements may not accurately predict device temperatures. For example, Kumar et al. teaches measuring of the air mass flow rate and temperature of a forced air system. Assumptions are made in that all the heat flux flows through the heat sink, which the measured air is passing over, and that the measured air properties are homogenous. These assumptions may not be reasonable as there may be other conductive, radiative and convective thermal dissipation paths present. In addition, the cooling efficiency of the heat sink may change over time. A buildup of dust, a blockage of the air, or a change to the thermal impedance properties of the interfaces material between the semiconductor device and the heat sink all contribute to a different thermal gradient between the measured point X and the junction temperature. None of these contributing factors will be indicated by the stated measurement of the air properties. Anticole teaches measuring the temperature remote, but close to, the device. Like many of the other inventions, Anticole assumes that the thermal resistance is known. The difficulties with this assumption are stated above. In addition, Anticole also assumes that the thermal time constant between the measurement point and the heat source is known. In this way, as the power delivered to the device is being monitored, the temperature at the source can be estimated more accurately and in a more timely manner during transient conditions. The difficultly is that the thermal time constants can also change over the life of a device. For example, assume that the thermal transfer material between the device and its primary thermal dissipation path decreases its thermal impedance over time. Because of this, the aggregate thermal conductivity of the system increases. As the thermal conductivity increases, the time constant decreases. In the Anticole invention, an overestimated time constant will result in an overestimated junction temperature.
Another technique that is taught is very similar to the external sensor technique above (e.g., U.S. Pat. No. 3,480,852-Hung, U.S. Pat. No. 4,823,290-Fasack et al. and U.S. Pat. No. 3,651,379-Moisand et al.). Multiple sensors are used including at least one ambient temperature sensor. The output from these devices are compared against limits and a control circuit responds accordingly. The theory has it that this device can predict the junction temperature more accurately and more rapidly in transient situations than the solution that has a single sensor alone. In practice however, measuring the ambient at a point source only determines the ambient temperature at that particular point. As mentioned, in many electronic devices, ambient temperatures can vary radically from point to point. Only if the sensor were located in a statistically average point would this solution be able to accurately determine the junction temperature. Any location other than this ideal point would generate an overestimation or an underestimation of the junction temperature.